How to Turn an IKEA Kallax Into a Window Seat (Without the DIY Cushion Headache)

The Kallax-to-window-seat hack is one of the most popular IKEA projects out there — flip the shelving unit on its side, add a cushion, done. This guide covers the build, the exact cushion dimensions for every Kallax size, what the DIY cushion route actually costs, and how to get one that fits without the trial and error.

This is one of the most reliably good IKEA hacks out there, and it earns the popularity. A Kallax shelving unit, laid on its side under a window, instantly becomes a bench with built-in storage underneath — baskets, bins, books, whatever needs a home. The structural part of this project is almost embarrassingly simple. The part that trips people up, consistently, is the cushion.

Assemble the Kallax unit according to IKEA’s instructions, then lay it on its side rather than standing it upright. The cube openings that would normally face outward now face up, which is what creates the bench seat surface and the storage cubbies in one move.

Position it beneath the window, against the wall. If the unit will see regular use as seating, anchor it to the wall using basic safety hardware — an anti-tip kit or L-brackets into a wall stud — both for stability and so it doesn’t shift when someone sits down at the edge. Many people add a thin plywood or MDF top panel across the full length before the cushion goes on, which spreads weight more evenly across the cube walls and gives the cushion a flat, continuous surface to sit on rather than resting directly on the cube openings.

That’s genuinely most of it. Kallax units are rated for roughly 29 lbs per cube in their designed orientation, but once laid flat with a top panel distributing the load, they reliably handle adult sitting weight — plenty of real-world builds report years of daily use with multiple people sitting at once without issue.

Here’s where most tutorials get vague, and where this guide is going to be specific. IKEA sells Kallax units in several configurations, and each one needs a different cushion footprint once laid on its side. These are the standard exterior dimensions for the most common configurations:

These reflect the Kallax unit’s own footprint when laid flat. If you’ve added a plywood top that extends past the unit’s edges, or built out the depth with additional framing, measure your actual finished surface rather than relying on these baseline numbers — plenty of real builds extend beyond the stock Kallax footprint for a deeper, more comfortable seat.

Two important notes the standard numbers don’t capture. First, if you’ve combined multiple Kallax units side by side — a very common approach for wider window seats — measure the actual combined width yourself, including any spacers or framing between units, rather than just adding the individual unit widths together. Small gaps and connecting hardware change the total. Second, many builders extend the depth beyond the stock 15½” by adding a front lip or plywood overhang for a more comfortable, deeper seat — if you’ve done this, your actual cushion depth needs to reflect that addition, not the stock Kallax measurement.

Most Kallax window seat tutorials walk through making your own cushion, and it’s a legitimate option — but it’s worth knowing what you’re signing up for before you start, because the real experience reported by people who’ve done it is more involved than the simplified version usually suggests.

The process: cut a piece of plywood or MDF to your bench’s exact surface dimensions (most hardware stores will make the first few cuts free), layer high-density foam on top (1 to 3 inches, cut to match the panel), add a layer of batting or quilt batting over the foam for a fuller look, and wrap the whole assembly in fabric, stapling it to the underside of the panel.

The complications that show up in practice: foam doesn’t always come in the length you need — people building the popular 95-inch combined Kallax bench have specifically run into trouble finding foam that long and have had to piece together sections or substitute mattress toppers cut to size. Corners are consistently described as the trickiest part to staple cleanly, with most builders folding and tucking to hide the bulk rather than achieving a fully clean finish. And for anyone wanting the bench to double as occasional sleeping space — a common reason for building a wider Kallax window seat in a kid’s room — the cushion needs to be thick and supportive enough for that use, which pushes costs up further.

One detail that’s easy to miss: Kallax tops are smooth laminate, which means a cushion sitting on top without any anchor will slide. Most successful builds add hook-and-loop strips between the cushion’s underside and the bench top to keep it in place — a small detail that’s worth planning for regardless of which cushion route you take.